Responses:

@raisingagent it takes advantage of javascript's automatic typecasting and quirks. JSFuck has a bit of an explainer at the bottom of the page, but the essence is that various empty things (such as an empty array, []) can be cast into other types (such as a number, +[], or a boolean, ![]). By combining these and the functions that exist as part of the type prototypes, you can do anything you want with only []()!+.